The present invention relates to a method for monitoring a web of material.
U.S. Pat. No. 4,344,073 describes a limit switch for watching for a tear in a web of material. This limit switch is arranged together with a compensating roller. In case of a tear in the web of material, the position of the compensating roller is changed and this change in position actuates the limit switch.
A measurement of the tension of the web of material, by operation of the compensating roller, is not provided.
DE 43 11 774 A1 discloses a safety device for a rotary printing press. Here, cylinders are provided which roll up the web of material in case of a tear of the web. At least one such cylinder is associated with a load sensing bearing. This load sensing bearing detects the pressure by the web of material rolled up between two cylinders, but does not detect the web tension.
DE 43 04 306 A1 describes a method for controlling the tension applied to a web in a web-retainer system. Here, an output characteristic of a tension device is used for controlling the torque of the retaining roller.
DE 40 39 108 C1 shows a device for preventing damage to the print units in case of a web tear. A measuring value sensor is installed on a chilling unit roller for measuring the web tension.
The object of the present invention is directed to providing a method for monitoring a web of material.
In accordance with the present invention, this object is attained by determining a gradient of a course of a web tension. A web tear is detected on the basis of a comparison of this gradient with a threshold value.
The advantages which can be achieved by the present invention reside, in particular, in that the measuring roller which detects the web tension is used as the detector for web tears. Fluttering of the web, or the change of the type of a web of material, does not result in an erroneous detection, as compared to prior art sensors measuring the distance from the web of material. Very short reaction times are possible because of the use of a stationary measuring roller in contrast to compensation rollers, since almost no moments of mass inertia need to be overcome.
The arrangement of the measuring roller of the present invention is independent, to a large extent, of the location of the web tear, since a drop in the web tension spreads very rapidly in the press.
The use of a percental threshold value for the permissible change of the web tension assures an evaluation of a web tear which is independent of production conditions.